1. Field of the Invention
The invention relates to the preparation of lubricating greases which can be used in the automotive and aerospace industry. More particularly, there is provided an improved method for preparing dry powder lithium carboxylates for use in lithium based greases.
2. Description of the Prior Art
Lithium soaps and complex soaps used as thickening agents for lubricating greases are generally prepared by reacting lithium hydroxide monohydrate or other lithium bases with aliphatic monocarboxylic and/or dicarboxylic acids which may be saturated or unsaturated, straight or branched chain, and may be hydroxy substituted. Preferably, these acids contain about 6 to 30 carbon atoms and more preferably, from about 6 to 18 carbon atoms. It has long been known that a grease comprising a lithium soap of hydrogenated castor oil, or the lithium soap of 12-hydroxy stearic acid provide greases with exceedingly high m mechanical stability and excellent water resistance.
In general, any of the conventional lubricating oils such as mineral, animal, vegetable or synthetic lubrication oils, may be employed as the grease base stock. These lubricating oils have a viscosity in the range of about 35 to 200 SSU at 210xc2x0 F. Mixtures of lubricating oils may also be effectively utilized. The grease compositions will usually contain about 70 to 95 weight percent, preferably about 80 to 95 weight percent, based on the total grease of the lubricating oil base. The lithium soap content may range from about 5 to 30 weight percent, preferably about 8 to 20 weight percent based on the total soap composition.
The prior art lithium greases are prepared as follows. The 12-hydroxy stearic acid or hydrogenated castor oil is dissolved in lubricating oil while heating to about 125 to 175xc2x0 F. at atmospheric pressure. Lithium hydroxide is dissolved in water and then added to this solution at about 180-190xc2x0 F., in a stoichiometric amount for complete reaction saponification of the 12-hydroxy stearic acid and to provide a mixture of the lubrication oil and lithium soap; heating the mixture carefully (due to foaming) to eliminate the water until it is uniform at a temperature from about 350xc2x0 F. to 430xc2x0 F.; rapidly cooling the mixture to about 300xc2x0 F. or below by quenching with additional lubricating oil and finally incorporating the remainder of the lubricating oil into the grease composition. The lithium soap or complex was prepared in situ and not isolated.
The invention relates to the preparation of lithium carboxylates having at least four carbon atoms in a process whereby the product can be recovered without filtration.
According to the process, either solid or molten carboxylic acid or glyceride thereof having at least four carbon atoms is added to a heated reaction vessel with high shear stirring. A stoichoiometric amount of lithium hydroxide monohydrate crystals or a concentrated solution thereof is added with heating and stirring. The heating and stirring is continued until the reaction is complete and the water driven off and the product is essentially dry.
The resulting product can be used in the manufacture of compositions containing lithium carboxylates such as soaps, oils and greases.
One or more monobasic or polybasic acids can be utilized.
It is therefore a general object of the invention to provide a process for preparing lithium carboxylates without requiring a filtration step.
It is a further object of the invention to provide lithium carboxylates salts or soaps thereof which can be used to prepare additives, oils or greases.
According to the invention lithium carboxylates having at least four carbon atoms can be prepared in a dry powder form without the requirement of filtration or expensive separation so as to be useful in oils, soaps and greases. The preparation is carried out using the acids or glyceride esters of the acids including mixed mono-, di- and triglyceride esters derived from animal fats, and oils and/or vegetable fats and oils. Such materials comprise fatty acid esters having 10 to 20 carbon atoms and are well known for use in soap making processes.
By xe2x80x9cglyceridesxe2x80x9d herein is meant organic acid esters of glycerol. The term xe2x80x9cglyceridesxe2x80x9d encompasses mono-, di- and triglycerides, since glycerol is a trihydric alcohol which can be esterfied on any, or all, of the three hydroxyl groups. Triglycerides constitute the major components of naturally occurring fats and oils which are typically used as starting materials in soap making processes.
By xe2x80x9canimal or vegetable fats and oilsxe2x80x9d herein is meant the organic acid glyceride materials which can be secured from a wide variety of sources. Specific, non-limiting examples of such materials include lard, tallow, coconut oil, palm oil, various by-products from animal rendering operations, oils from oleaginous seeds such as the soybean, sunflower seeds, and the like, cottonseed oil, etc. Typical listings of such materials are widely available, and all such glyceride mixtures are useful in the present process.
It has also been found that one can efficiently react a carboxylic acid or dicarboxylic acid or a mixture thereof, for example, 12-hydroxystearic acid directly with lithium hydroxide in a reactor equipped with high shear stirrers which stir the entire contents of the reactor. This is accomplished by adding a concentrated lithium hydroxide solution (preferably saturated to reduce the amount of water), or even lithium hydroxide monohydrate with essentially no oil present. The oil slows down the reaction and produces foam and ultimately a paste. In fact it has been found that a coarse to fine powder is produced upon complete reaction and removing substantially all of the water. this performed lithium ester powder can be conveniently discharged from the reactor and shipped to another location and incorporated into a grease formulation to produce a satisfactory lithium grease equivalent to that which is made by reacting the same carboxylic acid in situ with the lithium hydroxide in the base oil. This avoids the 3 to 4 hour reaction time normally required for the lithium hydroxide to react with 12-hydroxystearic acid (HSA) or hydrogenated castor oil (HCO) in the presence of base oil and this avoids controlling the foaming due to water which must be removed.
Crystalline LiOH.H2O and HSA can be reacted together starting at room temperature and the temperature taken up to 110xc2x0 C. and held for 15 to 30 minutes with good stirring. A saturated aqueous solution of LiOH (about 10.8%) can be added directly to molten HSA at 100-120xc2x0 C. to form the desired lithium 12-hydroxy stearate (LHS). The conditions can be varied over a wide range. The key is to react the HSA molten (above 86xc2x0 C.) with lithium hydroxide with the amount of water ranging from the monohydrate up to a 5% by weight LiOH solution with essentially no oil present (less than 30% by weight of the HSA). After the reaction is complete, up to 10% oil can be added to control dustiness and yet retain good flowability.
In the case of hydrogenated castor oil (HCO), the saponification of this triglyceride also proceeds as almost readily as the reaction with the HSA in the above described processes and produces the same resulting lithium 12-hydroxystearate which contains about 8% glycerine which is also formed in the saponification. This material can be used directly in making lithium greases which currently use HCO.
The preferred process conditions for making the lithium salts of carboxylic and polycarboxylic acids containing 4 or more carbon atoms including HSA, HCO and other triglycerides and mixtures thereof is the following:
a) Charge the stirred reactor with the requisite amount of acid or triglyceride material and heat up to 100-110xc2x0 C.;
b) Add the stoichiometric amount of a freshly filtered saturated lithium hydroxide solution (about 10.8% by weight) slowly over 30 minutes while maintaining the temperature between 90 and 110xc2x0 C. Stir and continue heating to drive off the water at 100-120xc2x0 C. Vacuum can be applied to assist and speed up the removal of the water. Vigorous stirring is maintained throughout this entire sequence. The mixture becomes very doughy just after neutralization is complete and remains this way until about half the water is removed. It then changed slowly into a powder on further drying.
c) The powder after total water removal is then cooled and discharged into a shipping container.
The product should be less than 0.5% water. The product may be adjusted to neutral to slightly acid or even slightly basic during the neutralization step depending on the grease formulation for which it is to be used. The yield is quantitative. The handling has been minimized in this process and the water removal step has been optimized using the least amount that is practicable. The present process avoids the use of a filtration or spray drying step which is time consuming and messy with these wax-like products.
Certain lubricant manufacturing operations require a preformed lithium soap such as synthetic lubricants used in aircraft since their base stock oils are high molecular weight esters or organic phosphates which can not tolerate the presence of lithium hydroxide. Therefore the lithium soap must be preformed.
In the case of lithium complex greases, these are complex mixtures of lithium 12-hydroxystearate and a dibasic acid such as azelaic acid, sebacic acid, succinic acid, etc., which are made in the grease all together. The ratio of HSA to azelaic acid or other dibasic acid weight ratio in these complex greases is usually 3/1 to 0.5/1 by weight. These complex lithium salts have never been preformed and dried before. It has been found that these complexes do form and separately, the corresponding lithium salts of the dibasic acids alone, can both be readily made by the present process. These preformed complexes readily form a satisfactory lithium complex grease.